Many of today's more sophisticated machine tools require little, if any, human operator intervention. As a result, there is an increasing need for monitoring systems that operate to check various parameters such as power consumption, force, torque, vibration, acoustical emission and the like during the machining process to ensure that there has not been some detrimental occurrence that may have otherwise been detected by the human operator.
Various monitoring systems for machine tools are known in the art. One such monitoring system is sold by the assignee of the present invention under the trademark "Tool Sense". U.S. Pat. No. 4,279,013 which is also assigned to the present assignee is representative of patent literature on the subject of machine tool monitors. In general, these monitoring systems are used to compare real time sensed parameters with preset limits. For example, the parameter may be power and the monitoring system is adapted to compare instantaneous power consumption of the machine with high and low limit values. If these limit values are exceeded then the monitoring system provides an output signal which is used to alter the machining operation so as to avoid catastrophic problems. Some of the more sophisticated monitoring systems can monitor more than one parameter at the same time.
Typically, a machine tool is used to remove the metal from a workpiece in a part cycle which includes several different operational stages. Some of the known machine tools such as the more sophisticated machining centers will operate in a number of different operational stages during a part cycle such a milling, drilling, boring, facing, spotting, counterboring, threading and tapping on four or more faces of the workpiece in a single setup. Machines of this type often have automatic tool changing capabilities in which the tools are stored in a magazine or matrix and are accessed by tool changing mechanisms. Upon completion of an operational stage, a new tool from the magazine is automatically interchanged with the tool in the machine spindle and the tool taken from the spindle is returned to the storage magazine.
It can be appreciated that there is a need to change the limit values depending upon the operational stage that the machine is undergoing. For example, it can be expected that more power will be used during a particular milling operation than is normally used for another type of operational stage. Therefore, in order to ensure accurate monitoring it becomes necessary to use different limit values during different operational stages. In the past, this has been accomplished by storing limit values in blocks or sections in a memory. Each section is associated with a particular operational stage. The limit values can be manually programmed by an operator or chosen automatically by using a so-called "learn" mode where a successful part cycle has been performed on a workpiece.
Unfortunately, it has been difficult to know when to use each section's parameters during the comparison process while the machine is operating. In other words, it is often hard to synchronize the variable protection limit data with the operational stage that the machine is undergoing. One attempt to accomplish this synchronization process is to have the machine controller generate certain codes (often referred to as "M" codes) at various times during the part cycle program. Unfortunately, these codes are not universally available from all machines and, if available, only a limited number of them can normally be used for this purpose.